The present invention relates to a white balance control system, and more particularly, to a system which provides an automatic correction of a color temperature in a color video camera.
In a color video camera, an adjustment of a white balance is required in order to produce a color picture of a high quality. If the color temperature of a light source changes, a corresponding change occurs in the white balance. Accordingly, in addition to the provision of a television camera tube or camera element, it has been proposed to provide a sensor in connection with the correction of the color temperature of a color video camera which determines the color temperature of a light source associated with an object being imaged. A white balance control system is provided which utilizes an output from a sensor in an arithmetic operation to adjust the white balance automatically.
FIG. 1 is a block diagram of a conventional white balance control system. Light emitted from a light source 1 (more generally an object being imaged) passes through the optical lens 2 of a color video camera to be imaged on a camera element 3. The camera element 3 produces three electrical signals for the red (R), green (G) and blue (B), respectively, which signals are amplified by an R signal amplifier 4, a G signal amplifier 5 and a B signal amplifier 6, respectively. In the arrangement shown, the amplification degree of the amplifiers 4, 5 and 6 is controlled by a white balance adjusting circuit 7. An R/B photodetector 9 formed by a light receiving element having a color filter of a spectral response which is identical to that of the camera element 3 is located so as to receive light from the light source 1 (an object being photographed), with an opal diffuser plate 8 being disposed in front of the detector 9. Various colored lights from an ordinary scene, when added together, infinitely approach the white, and accordingly, the purpose of the opal diffuser plate 8 is to diffuse reflected light from an object being photographed to form an additive sum of light transmitting therethrough over the entire surface of the diffuser plate 8.
R and B signals from the photodetector 9 which are converted into photocurrents i.sub.R, respectively, i.sub.B, are fed to logarithmic amplifiers 10, 11, respectively, which produce out-put signals log i.sub.R, log i.sub.B. Outputs from the logarithmic amplifiers 10, 11 are fed to a differential amplifier 12 which produces an output signal equal to log i.sub.R -log i.sub.B =log (i.sub.R /i.sub.B) which is then fed to the white balance adjusting circuit 7. The output signal log (i.sub.R /i.sub.B) changes solely in accordance with a change in the color temperature of the light source 1, independently of the amount of light which is incident upon the photodetector 9, and hence the white balance adjusting circuit 7 controls the amplification degree of the R and the B signal amplifier 4, 6 in a direction to achieve a proper white balance. Specifically, if an object being photographed is a white paper, it is required that outputs from the color video camera maintain a faithful reproduction of the white when amplified by the amplifiers 4 to 6. To this end, the adjusting circuit 7 operates to change the amplification degree of the amplifiers 4 and 6 in accordance with the output signal log (i.sub.R /i.sub.B) from the differential amplifier 12, thus achieving a mixing of the signal levels of R and B, which are two of three primary colors R, G and B, at a proper proportion, which is a white balance control.
By utilizing the white balance control system mentioned above, a proper white balance can be maintained by detecting the R and B components of light from an object being photographed which is under illumination of sunlight or radiation from a tungsten lamp which can be approximated by the spectrum of radiation from a black body by means of the R/B photodetector 9, in a similar manner as by the television camera element 3. However, where the light source comprises a fluorescent lamp or the like, such source exhibits a peculiar spectrum which is distinct from that of the radiation from the black body, so that when a color video camera is used including circuitry of the type which controls the gain over R and B signals in response to the detection of R and B components of incident light, an offset in the adjustment of the white balance is caused, preventing the maintenance of a good white balance from being achieved.
To cope with this problem, there has been proposed a white balance control system (see Japanese Laid-Open Patent Application No. 127,376/1982) including a sensor capable of detecting the spectra of three primary colors R, G and B and also including another sensor which is capable of detecting a distinct spectrum. However, this circuit arrangement requires the provision of a plurality of sensors since there are a variety of types of fluorescent lamp sources. If it is desired to cover every kind of lamp source, an increased number of sensors must be used, resulting in a complex circuit arrangement and an increased cost.
A human eye is capable of sensing a flickering in the radiation from a fluorescent lamp source, but a light sensitive element can detect such flickering in a more accurate manner. The flickering produced by a fluorescent lamp source has a period which is equal to the reciprocal of twice the frequency of a commercial power supply. It has been experimentally determined that the timing or phase of the emission differs between the three primary colors R, G and B. On the other hand, natural light or sunlight has no flickering, and a tungsten lamp exhibits a flickering which while having the same frequency has a substantially reduced magnitude as compared with that of a fluorescent lamp since the thermal time constant of the filament of the tungsten lamp has an increased value. Accordingly, a discrimination between a fluorescent lamp source and other light sources can be made by noting the magnitude of a specific frequency flickering from the source used (such flicker represents a ripple electrically, and hence will be hereafter referred to as a "ripple").